Damian Lillard of the Bucks leaves game against Pacers due to lower leg injury.

 Milwaukee Bucks star Damian Lillard will undergo an MRI on Monday to assess the extent of the left lower leg injury that forced him to leave Sunday night's 129-103 Game 4 loss to the Indiana Pacers in the first quarter.

Preliminary examinations suggest a potential Achilles tendon injury, sources told ESPN, confirming multiple reports.

"Honestly, it's not looking very promising," Bucks coach Doc Rivers said after the game.

Lillard was injured midway through the first quarter while trying to secure an offensive rebound. After tapping the ball away from a Pacers player, he fell to the floor and immediately grabbed his left leg, remaining down for the subsequent defensive possession.

He was helped to his feet during the next dead ball, but could not put weight on his leg and needed assistance to leave the court. He was escorted to the locker room, where the team ruled him out for the rest of the game.

"I knew it immediately," Rivers said. "I felt bad for him; the guy came back for his team, and I just felt for him."

Lillard left the arena in a walking boot and on crutches.

"When you see someone like that unable to walk on their own, you know it could be serious," Bucks star Giannis Antetokounmpo said. "Now, we just hope for the best."

Lillard exited with 5:57 remaining in the first quarter, with the Bucks trailing 15-12. Rivers acknowledged that the loss of Lillard was demoralizing for the team.

"Dame's one of those special guys," said Bucks guard Kevin Porter Jr. "We need him. Seeing him go down and not return, it's definitely a tough blow."

Milwaukee is now down 3-1 in the series against Indiana, with Game 5 scheduled for Tuesday in Indianapolis.

Lillard had missed the final 14 regular-season games due to deep vein thrombosis in his right leg, returning in Game 2 of the series after a month-long recovery. However, he had not been his usual self during the series, averaging just 10.5 points and 6.0 assists on 24% shooting (6-of-25) over the first two games. He missed both his shots in Game 4 before being injured.

"When a guy works so hard to get back on the court, and then something like this happens, it's really tough for him," said Bucks forward Bobby Portis. "It's tough for us too. We need him out there."

Following Lillard's injury, the Pacers went on a 10-3 run, taking a double-digit lead into halftime. They extended the lead in the second half for a 26-point victory, the second-largest home playoff loss in Bucks history, according to ESPN Research.

"This time of year, everyone's focused on the rivalry and competition, but at the end of the day, this league is a brotherhood," said Pacers center Myles Turner. "He's always been one of my favorite competitors. Thoughts and prayers go out to him. You never want to see something like that happen."

Milwaukee now faces the prospect of a third consecutive first-round playoff exit. As they await the MRI results on Monday, the Bucks are preparing to move forward without Lillard.

"We all know the deal," Antetokounmpo said. "The next guy has to step up. We’ve got to keep playing together, move the ball, and do what we can to win."